Many processes for producing a dispersion of particles of rutile titanium oxide are already conventionally known. The most general process is the one in which a titanium salt is hydrolyzed to prepare a slurry of hydrous titanium oxide, and the slurry is deflocculated with an acid or an alkali to obtain a dispersion. However, the particles of titanium oxide in the dispersion of rutile titanium oxide obtained in this conventionally known process have a D90 of 40 nm or more, and therefore they cannot be used in such an application of a resin molding or a coating agent for hard coating where the particles are required to have a high transparency.
Recently, some processes for producing a dispersion of fine particles of rutile titanium oxide are proposed. For example, a process is proposed in which titanium tetrachloride and hydrochloric acid are mixed in water having a temperature of 65 to 90° C., and the mixture is heated to a temperature of 65° C. to a boiling point of the resulting mixture to hydrolyze the titanium tetrachloride, thereby a dispersion of particles of rutile titanium oxide is obtained (Patent literature 1).
According to this process, a dispersion of fine particles of rutile titanium oxide having a BET specific surface area within a range of 50 to 300 m2/g can be obtained, but the process has a defect in which the particles of titanium oxide obtained have a low crystallinity.
In general, the higher the crystallinity of particles of rutile titanium oxide, the more effective the exhibition of various physical properties of the rutile titanium oxide such as a UV shielding property, photocatalytic activity, and refractive index, and hence it is preferred that the particles of rutile titanium oxide have a higher crystallinity. The increase of the crystallinity of particles of titanium oxide can be confirmed by increase of peak intensity in a powder X-ray diffraction analysis without being accompanied by particle growth. Furthermore, it is believed that the particles of rutile titanium oxide obtained by thermal hydrolysis of titanium tetrachloride are not increased in crystallinity because amorphous titanium compounds are intermixed therein, and that there are volatile components such as water which are contained in or adhere to the particles. For comparison of the amount of the amorphous titanium compounds, variations in weight loss on heating may be compared. It can be said that the smaller the weight loss on heating, the higher the crystallinity.
It is traditionally known that the crystallinity of rutile titanium oxide can be increased by calcining it, but when the oxide is calcined, the particles grow at the same time, and therefore the crystallinity cannot be increased while the particles remain fine. To be highly crystalline is traded off against to be fine in particle size.
As another process for producing a dispersion of fine particles of rutile titanium oxide, for example, a process is proposed in which fine particles of rutile titanium oxide which have been previously produced are dispersed in water (Patent literature 2). According to this process, however, the particles of titanium oxide dispersed in water have an average particle size at least of no less than 70 nm.